1. Field of the Invention
The present invention relates to a method of automatically correcting aberrations in a charged-particle beam and to an apparatus therefor.
2. Description of Related Art
In scanning electron microscopes and transmission electron microscopes, an aberration corrector is built in the electron optical system to permit high-resolution imaging and to enhance the probe current density. A system consisting of an aberration corrector having four stages of multipole units has been proposed as the first-mentioned built-in aberration corrector. Each multipole unit has twelve pole elements. Chromatic aberration is corrected by a combination of an electrostatic quadrupole mode of operation and a magnetic quadrupole mode of operation. Spherical aberration is corrected by four stages of octopole mode of operation. The principle is introduced in detail in H. Rose, Optik 33, Heft 1, 1–24 (1971); and J. Zach, Optik 83, No. 1, 30–40 (1989); J. Zach and M. Haider, Nucl. Instru. and Meth. In Phys. Res. A 363, 316–325 (1995).
The principle of the above-described aberration corrector is described briefly with reference to FIG. 8. In FIG. 8, an aberration corrector C is disposed ahead of an objective lens 7 and equipped with four stages of multipole units 51, 52, 53, and 54. Each of the multipole units 51–54 has twelve pole elements. Electric potentials 1, 2, 3, and 4 are applied to the multipole elements 51–54, respectively, to activate the electrostatic quadrupole elements. Exciting currents 5 and 6 are supplied to the second and third stages of multipole units 52 and  53, respectively, to produce magnetic potential distribution analogous to the electric potential distribution created by the potentials 2 and 3 and to produce a magnetic field superimposed on the electric field. Furthermore, electric potentials 11, 12, 13, and 14 are applied to the multipole units 51–54, respectively, to activate an electrostatic octopole element for producing an electric field superimposed on the electric field produced by the potentials 1–4 that are used to activate the quadrupole.
In actual instrumentation, electric fields produced by dipole-activating potential (acting as a deflector for axial alignment) and hexapole-activating potential (acting to correct second-order aperture aberration) are superimposed on the electric fields produced by the quadrupole-activating and octopole-activating potentials. Since these superimposed fields are hardly related directly with aberration correction for which the present invention is intended, they will not be described in detail below.
In the configuration of FIG. 8, with respect to a charged-particle beam B entering from the left side as viewed in the figure, a reference orbit for the beam B is created by the four stages of multipole units 51–54 and objective lens 7. The beam B is focused onto a specimen surface 20. In FIG. 8, X-direction orbit Rx and Y-direction orbit Ry of the beam B are both schematically drawn on the same plane.
The reference orbit can be understood as a paraxial orbit that is assumed when there is no aberration. That is, the Y-direction orbit Ry is made to pass through the center of the multipole unit 52 by the multipole unit 51. The X-direction orbit Rx is made to pass through the center of the multipole unit 53 by the multipole unit 52. Finally, the charged-particle beam B is focused onto the specimen surface 20 by the multipole units 53, 54 and objective lens 7. In practice, these need to be adjusted mutually for complete focusing. At this time, the dipole-activating potentials applied to the four stages of multipole units 51–54 are used for axial alignment.
Referring more particularly to FIG. 8, the charged-particle beam B in the X-direction orbit Rx is diffused by the multipole unit 51 acting like a concave lens. Then, the beam is focused by the multipole unit 52 acting like a convex lens and made to pass through the center of the multipole unit 53. Then, the beam is focused by the multipole unit 54 and moves toward the objective lens 7. On the other hand, the beam B in the Y-direction orbit Ry is focused by the multipole unit 51 and made to pass through the center of the multipole unit 52. Then, the beam is focused by the multipole unit 53. Finally, the beam is diffused by the multipole unit 54 and moves toward the objective lens 7. By combining the diffusive action of the multipole unit 51 acting on the orbit Rx in the X-direction and the diffusive action of the multipole unit 54 acting on the orbit Ry in the Y-direction in this way, the electron optical system can be operated like a single concave or convex lens.
Correction of chromatic aberration in the charged-particle beam B using the aberration corrector C is next described. To correct chromatic aberration in the system shown in FIG. 8, electric potential φq2 [V] acting as an electrostatic quadrupole element and magnetic excitation J2 [AT] (or magnetic potential) acting as a magnetic quadrupole element are adjusted such that the reference orbit remains unchanged. The whole lens system acts to correct the X-direction chromatic aberration to zero. Similarly, electric potential φq3 [V] acting as an electrostatic quadrupole element and magnetic excitation J3 [AT] acting as a magnetic quadrupole element are adjusted such that the reference orbit is not affected. The whole lens system acts to correct the Y-direction chromatic aberration to zero.
Correction of spherical aberration (correction of the third-order aperture aberration) in the charged-particle beam B is next described. Spherical aberration is corrected after X- and Y-direction chromatic aberrations are corrected. The X-direction spherical aberration in the whole lens system is corrected to zero by electric potential φ02 [V] acting as an electrostatic octopole element. The Y-direction spherical aberration is corrected to zero by electric potential φ03 [V] acting as an electrostatic octopole element.
Then, the spherical aberration in the resultant direction of the X- and Y-directions is corrected to zero by the electric potentials 11 and 14 for activating the electrostatic octopole elements. In practice, repeated mutual adjustments are necessary. Superimposition of the potentials and magnetic excitations for activation of quadrupole and octopole elements has been put into practical use by using a single unit having twelve pole elements and varying the potential or magnetic excitation applied to each pole of the twelve pole elements so as to synthesize dipoles, quadrupoles, hexapoles, octopoles, etc. This method has been introduced, for example, in M. Haider et al., Optik 63, No. 1, 9–23 (1982).
In particular, in an electrostatic design, a final stage of power supplies An (n=1, 2, . . . , 12) capable of supplying a voltage to twelve electrodes Un (n=1, 2, . . . , 12) independently is connected as shown in FIG. 9. Where a quadrupole field is produced, output voltages from a quadrupole power supply 10 are supplied to the final-stage power supplies An to obtain a field close to an ideal quadrupole field. If it is assumed that the output voltages from the final-stage power supplies An are proportional to the output voltages from the quadrupole power supply 10, the ratio of the output voltages from the power supply 10 assumes a value as given in the M. Haider et al., Id. Where an octopole field is created to be superimposed on this quadrupole field, output voltages from an octopole power supply 18 are added to the output voltages from the quadrupole power supply 10 and supplied to the final-stage power supplies An to obtain a field close to an ideal octopole field. Subsequently, based on a similar concept, a field on which a multipole field produced by operation of a 2n-pole element (n=1, 2, . . . , 6) is superimposed is obtained by activating the twelve poles formed on a single unit.
In a magnetic design, a final stage of power supplies B, (n=1, 2, . . . , 12) capable of supplying excitation currents to the coils on twelve magnets Wn (n=1, 2, . . . , 12) independently is connected as shown in FIG. 10. Where a magnetic quadrupole field is created, output voltages from a magnetic quadrupole power supply 15 are supplied to the final stage of power supplies Bn to produce a field close to an ideal magnetic quadrupole field. If it is assumed that the output currents from the final-stage power supplies Bn are proportional to the output voltage from the magnetic quadrupole power supply 15, the ratio of the output voltages assumes an exciting magnetic force ratio as given in the above-quoted M. Haider et al. reference. Superimposition of multipole fields other than a magnetic quadrupole field is not explained herein. However, a magnetic multipole field can be superimposed in the same way as in the electrostatic design, by adding voltages for multipole fields to the input voltage to the final-stage power supplies Bn. A yoke for magnetically connecting the outside portions of the magnets Wn is omitted in FIG. 10.
When electrostatic and magnetic designs are superimposed, a conductive magnetic material may be used so that the magnets Wn can act also as the electrodes Un. In this case, the coils on the magnets are disposed so as to be electrically isolated from the electrodes.
In the description given below, the 2n-pole elements are treated as if they were stacked on top of each other to simplify the explanation. In practice, superimposition of plural multipole fields on a dodecapole (twelve-pole) element mounted on a single unit is achieved by adding voltage signals as mentioned previously.
After end of correction of chromatic aberration in the charged-particle beam B, it may be necessary to correct the second-order aperture aberration by means of three or four stages of hexapole elements before the correction of spherical aberration is performed. This correction is made in the same procedure as in the aforementioned correction of spherical aberration. This second-order aperture aberration occurs depending on the mechanical accuracy of the aberration corrector. Normally, the amount of correction is small, and this aberration affects higher-order aberrations only a little within the scope of the present aberration corrector. Furthermore, the second-order aperture aberration is corrected within the aberration corrector. If the resultant magnification (described later) of the aberration corrector and the objective lens is varied, higher-order aberrations are affected little, though the resultant magnification is important in aberration correction. For these reasons, description of the correction of the second-order aperture aberration is omitted in the description of the prior art.
A method of detecting geometric optics aberrations up to the third order using a probe, especially in a scanning microscope equipped with a point light source, lenses, an object, and a detector, is known, for example, as described in Unexamined Japan Patent Number P2003-521801 (paragraphs 0006–0008, FIG. 1).
The aforementioned procedure of the prior art correction of aberrations in a charged-particle beam is complex. There is the problem that it takes a considerable time for an ordinary operator to master the technique and obtain high-resolution images.